1. Field of the Invention
The present invention relates to a method and circuit for controlling an LED load.
2. Description of the Related Art
Light Emitting Diodes (LEDs) are increasingly used in a wide range of applications. LEDs require current regulation instead of voltage regulation. An LED driving circuit, also referred to as LED driver, may be considered as a type of power conversion circuit that delivers a regulated current. However, if an LED, or a series of LEDs, requires a voltage of 12V and is connected to a 12 VAC-source, known LED drivers are highly inefficient as they need to be able to raise the voltage when the voltage provided by a rectified 12V AC-source is below 12V, and, at the same time, need to be able to lower the voltage when the voltage provided by a rectified 12V AC-source is above 12V in order to ensure that a constant current is delivered.
U.S. Pat. No. 7,276,861 describes a system and method for driving an LED in which the system includes a switching power converter that can be a step-up switching converter, also referred to as boost converter, or a step-down switching converter, also referred to as buck converter. The boost converter is used if the source voltage should be boosted. The buck converter is used if the source voltage should be decreased. Alternatively a buck-boost topology is used, i.e. a boost converter and buck converter are combined in a single circuit. The switching power converter includes an inductor, and a switch. The converter operates with an on-time phase when the switch is closed and an off-time phase when the switch is open. Energy is stored in the inductor during on-time of the switch, while during off-time of the switch, the energy is discharged into the LEDs. If both boost and decrease of voltage are needed, i.e. a buck-boost topology is used, the switching power converter comprises more components than a regular boost or buck converter, i.e. typically at least an additional switch and an additional diode.
Furthermore, the switching power converter comprises a current comparator to enable regulation of the length of the switch on time. By measuring the current through the inductor during off-time of the switch, a suitable on and off time of the switch may be determined. However, the need for a current comparator makes the circuit more complex and costly than necessary.
Hence, a circuit as described in U.S. Pat. No. 7,276,861 is relatively complex and costly to manufacture. Furthermore, a compact circuit is highly desirable, especially in applications where LEDs are replacing conventional lighting that does not require a driving circuit. Thus, there is a need for a simple low cost driver circuit with a minimum number of components.